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nucleophilic substitution

http://www.google.com/url?sa=t&rct=j&q=january%202010%20question%20paper%20f332%20ocr&source=web&cd=1&ved=0CCwQFjAA&url=http%3A%2F%2Fwww.ocr.org.uk%2Fimages%2F57915-question-paper-unit-f332-chemistry-of-natural-resources.pdf&ei=3NtyUcaBAcvWPPKygbAK&usg=AFQjCNErxQWwNPmE9RLVXjjg1wS_9KLqNA&sig2=JyPddmoDoYvqiUB5RwRtLg&bvm=bv.45512109,d.d2k

I need help on question 2bii. The answer is nucleophilic and substitution, but why? The Br-H molecule is polarised with bromine having a slightly negative charge and hydrogen having a slightly positive charge. In the methanol molecule's C-O bond, the C is sightly positively charges and the O is slightly negatively charged. So the Br in Br-H is attracted to the C in methanol while the O is repelled. So bromine joins onto carbon O-H goes off and reacts with hydrogen to make water, forming bromomethanol. Is this a right explanation

Btw, why can't the substitution be electrophilic? Is it because there's no double bonds in methanol, so there is not a region of very high electron density,
Seems right to me! Electrophilic addition is all about the addition of a halogen into the compound I think because of the whole "rich in electron density" I think, sorry I'm not too confident about organic chemistry :smile:
Reply 2
Avatar for illusionz
illusionz
20
Original post by krisshP
http://www.google.com/url?sa=t&rct=j&q=january%202010%20question%20paper%20f332%20ocr&source=web&cd=1&ved=0CCwQFjAA&url=http%3A%2F%2Fwww.ocr.org.uk%2Fimages%2F57915-question-paper-unit-f332-chemistry-of-natural-resources.pdf&ei=3NtyUcaBAcvWPPKygbAK&usg=AFQjCNErxQWwNPmE9RLVXjjg1wS_9KLqNA&sig2=JyPddmoDoYvqiUB5RwRtLg&bvm=bv.45512109,d.d2k

I need help on question 2bii. The answer is nucleophilic and substitution, but why? The Br-H molecule is polarised with bromine having a slightly negative charge and hydrogen having a slightly positive charge. In the methanol molecule's C-O bond, the C is sightly positively charges and the O is slightly negatively charged. So the Br in Br-H is attracted to the C in methanol while the O is repelled. So bromine joins onto carbon O-H goes off and reacts with hydrogen to make water, forming bromomethanol. Is this a right explanation

Btw, why can't the substitution be electrophilic? Is it because there's no double bonds in methanol, so there is not a region of very high electron density,


HBr is a strong acid (it's similar to HCl, but actually stronger). So it dissociates in polar solvents, and your alcohol is protonated by the H+ produced by HBr. This forms R-OH2+, which has a positively charged leaving group. Then the Br- attacks the carbon, substituting for water.

The reaction does not occur until the alcohol is protonated.

It is nucleophilic substitution because the bromine is acting as a nucleophile, and it is a substitution reaction.
Reply 3
Avatar for illusionz
illusionz
20
Original post by VK96
http://chemwiki.ucdavis.edu/Organic_Chemistry/Reactions/E1_Reaction/Carbocation_Rearrangements
look there and go down to the hydride shift, it explains it in steps. See if that helps.
It uses a more complicated molecule but try to apply what it says to a methanol molecule.


Hydride shift is not relevant in this case because there is only 1 carbon, and the reaction is SN2 not SN1.
Reply 4
Avatar for VK96
VK96
10
Original post by illusionz
Hydride shift is not relevant in this case because there is only 1 carbon, and the reaction is SN2 not SN1.

Ah I see, sorry about that :P
Reply 5
Avatar for krisshP
krisshP
OP
14
I kinda get it now after reading
http://www.chemguide.co.uk/mechanisms/nucsub/whatis.html#top

But how would I know just from an equation if there's electrophilic addition or nucleophilic substitution? Do I just see if there's a double bond for between carbons or what?
Reply 6
Avatar for illusionz
illusionz
20
Original post by krisshP
I kinda get it now after reading
http://www.chemguide.co.uk/mechanisms/nucsub/whatis.html#top

But how would I know just from an equation if there's electrophilic addition or nucleophilic substitution? Do I just see if there's a double bond for between carbons or what?


Basically you just need to know the mechanism of the reaction. You can't tell from the equation.
Reply 7
Avatar for krisshP
krisshP
OP
14
Original post by illusionz
Basically you just need to know the mechanism of the reaction. You can't tell from the equation.



Original post by VK96
Ah


Thanks for the help :smile:

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